Nicolas Carvalho Ochoa/Getty

NICE, France—The Barcelona city center was in lockdown late Thursday afternoon after a large white van plowed into pedestrians in the historic Las Ramblas district, killing at least 13 people and injuring more than 100, with the death toll expected to rise according to authorities.

The attacker, believed to be armed, was initially reported to be holed up in a local bar and possibly holding hostages but there was no confirmation from the police. By early evening, authorities confirmed at least two suspects had been arrested after the attack, and according to local press reports one suspect may have been shot.

The so-called Islamic State claimed a connection with the slaughter through its Amaq propaganda outlet, but offered nothing to substantiate what may have been an opportunistic statement.

Spain's Guardia Civil identified the man who had rented the truck as Driss Oukabir, originally from Marseilles, France, but there is no confirmation that he is one of those arrested or is implicated in the actual attack. And indeed a man was reported to have gone to a police station saying that he had seen his photograph on news reports, identified himself as Oukabir, and said his passport had been stolen.

In what may be a related incident, a car ran into two police officers from the Catalan security forces at a checkpoint on the outskirts of Barcelona, and one of them was lightly injured. According to sketchy reports on the website of La Vanguardia, a leading newspaper in the Catalan capital, there was an exchange of fire and the man trying to run through the checkpoint was shot, possibly killed.

According to La Vanguardia, the people involved in the attack—it is not clear how many there were—had rented two small trucks. Both were found, one of them parked in front of a Burger King, and both were examined by the bomb squad for possible explosives. None were reported found.

Earlier, a graphic video posted on Facebook showed a number of victims of the truck attack lying either dead or unconscious along the wide, tree-lined boulevard that was stained with blood. Other victims managed to sit up and were being tended to by medics or passersby.

The attack took place in the stately, elegant Las Ramblas section of the city during the height of the summer tourist season in Spain and had eerie parallels to last year’s truck attack in Nice, France, when 86 people died and hundreds were injured. Since then, we have seen vehicle attacks in Berlin, London, Stockholm, Israel, and in the United States in Charlottesville, Virginia.

Eyewitnesses reported that the van jumped the sidewalk in a pedestrian zone in Las Ramblas and careened from side to side as it barreled into tourists and local resident. In all it traveled 600 meters, more than a third of a mile from Plaza Catalunya to the mosaic by Joan Miró at Carrer Hospital.

U.S. Secretary of State Rex Tillerson said the incident in Barcelona “has the hallmarks it appears of yet another terrorist attack,” and said U.S. diplomats are “currently assisting Americans in Barcelona who are affected.”

“Terrorists around the world should know the United States and our allies are resolved to find you and bring you to justice,” he added, in remarks to the press in Washington, D.C., Thursday.

Spain was the target of the worst jihadi terrorist attack in European history in March 2004, when multiple bombs went off at Madrid's main commuter train killing 191 people. After a long lull, as The Daily Beast reported last year, tensions had been rising dramatically and there was a widespread feeling in Barcelona—and other Spanish cities—that despite heavy police patrols something like this might happen.

Amateur video showed paramedics tending to wounded people on the street while others were taken on stretchers into waiting ambulances.

Barcelona police took to Twitter to ask social media users not to tweet out graphic pictures of the dead and wounded. Guns drawn, police could be seen on Spanish television patrolling the area looking for the suspects.

Reports from Barcelona media indicated the attack occurred around 5:25 p.m. local time when the van plowed into people on the Ramblas’ main promenade. The driver of the van, who had what was described as a “long gun” and wearing a blue and white striped shirt, escaped. Many people took refuge in the shops along the boulevard.

In the immediate aftermath, some of the injured were covered with parasols from a coffee shop, one eyewitness said.

Restaurant worker Antonio Vito told Spanish media that he was speaking to two customers when he saw a large white van running at top speed “between 80 and 100 kilometers per hour” on the central Paseo de la Rambla. Vito said there was a stampede of people trying to escape the area after the attack.

Stanna Matias-Capet, another eyewitness, said the driver of the van wore sunglasses, had a beard and brandished a Kalashnikov.

A girl named Alba told a Spanish reporter that police came into the restaurant where she was eating and lowered the blinds so “nobody can lift them. They are looking for (the suspects.” Alba said two German girls sought refuge in the restaurant and could not “stop crying” out of fear over the attack.

Mayor Ada Colau, who was away on vacation, has returned to the city where many events for Thursday night were cancelled.